Seeking advice on the issue of inaccurate accumulation values in flow meters
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The company is involved in an energy metering project for a steel plant; Phase 1 has been completed, and Phase 2 is set to begin shortly. The flow meter in the first stage displays both instantaneous and cumulative values; the instantaneous signal is uploaded to the DCS system, while the cumulative value is calculated by the upper-level computer based on the uploaded instantaneous values. According to the owners, the cumulative values calculated by the host computer in Phase 1 are inaccurate, showing a significant difference from the cumulative values displayed on-site. Therefore, they require that the flow meters used in Phase 2 have the capability to upload the cumulative values from the on-site instruments; the host computer should not need to use algorithms for accumulation, but rather should display directly the cumulative values from the on-site instruments. I would like to ask the experts here: what are the reasons for the inaccuracies in the cumulative values calculated by the host computer? Is it due to the cumulative algorithm? Is there an error inherent in the accumulation itself (a significant error)? What is the difference between the cumulative value calculated by the field instruments and the cumulative value calculated by the host computer? Is it indeed more accurate than accumulation using a host computer? Please give me an answer, guys. Thank you! !
The instantaneous values uploaded to DCS cannot be analog signals; they can only be used as a reference. If pulse signals are used instead, calculations can be made with high precision. Of course, MODBUS communication via RS485 can be employed to keep things in line with what’s available on-site, and most flow meters provide this interface.
Thank you, sir! But if it is changed to a pulse signal, would it be necessary to add an additional DCS input module on top of the existing system, specifically for the transmission and calculation of accumulations? Generally, flow meters have the option of using the HART communication protocol for output, so I am considering adding HART communication functionality to transmit the cumulative volume. But to save costs back then, the HART function was not chosen; would adding HART communication now pose many difficulties in terms of investment and installation? Do ordinary flowmeters come with a MODIBUS interface? So, which of these three options is more cost-effective and convenient to modify? I also want to ask how the accumulation of instantaneous values is calculated. Why is such an accumulation definitely inaccurate? And why is accumulation using pulse signals accurate? Could you explain the principle behind it in detail? Is the accuracy of the cumulative value displayed by the on-site instruments due to the use of pulse signals for accumulation? I hope an answer can be provided; thank you all! !
The algorithms for on-site accumulation and DCS accumulation are different, and it’s normal to see discrepancies over time; I think neither of these two values can be considered reliable.
I don’t know why:lol :lol
The original poster has raised a very good question. Personally, I have always believed that cumulative calculations on the host computer will inevitably lead to errors; in comparison, the cumulative calculations performed by the instrument itself are more accurate, though this is not accuracy in a strict sense either. What we do now is essentially transmit the cumulative signals (pulse quantities) from the field instruments to the host computer for display. The prerequisite is the dashboard belt feature.
The cumulative value should be obtained from a single source and displayed in multiple places; instruments are more accurate than DCS systems. Even if DCS systems use pulse measurement, errors will occur over time compared to the values shown on the instruments. This will make users skeptical, as even accurate instruments have errors; therefore, such issues should be taken into account during the initial setup. :lol
Primary meters, secondary meters, and DCS systems all have their own maximum values for cumulative amounts, such as 0-99999, etc.; if these values are exceeded, re-cumulation is required! Is it caused by this reason?
I’m not sure which DCS you are using; in the configuration with Lishí, there is an instrument factor for cumulative values, and if this value is set incorrectly, it will affect the cumulative figures. It would be better if the cumulative value of the instrument could be transmitted to the DCS. Could someone explain one that can transmit both cumulative values and instantaneous values?